1. Field of the Invention
This invention generally relates to liquid crystal display (LCD) and integrated circuit (IC) fabrication and, more particularly, to a silicon film and fabrication process to laser irradiate silicon film in making polycrystalline silicon thin film transistors (TFTs) for Active Matrix (AM) LCDs.
2. Description of the Related Art
Lateral crystallization by excimer-laser anneal (LC-ELA) is a desirable method for forming high quality polycrystalline silicon films having large and uniform grains. Further, this process permits precise control of the grain boundary locations.
FIGS. 1a through 1d illustrate steps in an LC-ELA annealing process (prior art). As seen in FIG. 1a, initially amorphous silicon film 100 is irradiated by a laser beam that is shaped by an appropriate mask to an array of narrow “beamlets”. The shape of the beamlets can vary. In FIGS. 1a-1d, each beamlet is shaped as a straight slit of narrow width, approximately 3-5 microns (μm). This slit is represented in the figures as the two heavy lines. The width of the slit is the distance between these two lines. This width can vary, but ultimately it is dependent upon the attainable lateral growth length (LGL), which is defined as the distance crystals can grow laterally (inwardly) from the edges of the irradiated area. Typically, the beamlet width is designed to be slightly less than twice the corresponding LGL.
The sequence of FIGS. 1a-1d illustrates the growth of long polysilicon grains by LC-ELA process. A step-and-repeat approach is used. The laser beamlet width (indicated by the 2 parallel, heavy black lines) irradiates the film and, then steps a distance (d), to point 102, smaller than half of the lateral growth length (L); i.e. d<L/2. Using this step-and-repeat process, it is possible to continually grow crystal grains from the point of the initial irradiation, to the point where the irradiation steps cease.
FIG. 2 is partial cross-sectional view of FIG. 1a illustrating the surface topography of laser-irradiated domains (prior art). After the completion of the lateral growth, the two crystal fronts meet at the center of the domain where they form a “boundary” between the two crystal regions developing from each opposing edge of the domain. As a result of the grain boundary formation, a “ridge” 102 develops at the surface of the film at the boundary, corresponding to the planned congruence of the two crystal fronts. Since the substrate steps under the beam a distance of d, where d is less than L/2, the ridge 102 is irradiated is a subsequent shot. This ridge 102 remelts and locally planarizes. However, as part of the same process, another ridge is formed at a new location. Therefore, the ridge location will “march” across the substrate in response to the scans under the beam.
FIG. 3 is the silicon film 100 of FIG. 2 schematically illustrating the evolution, or the ridge 102 motion pattern after 1, 2, 3 and “n” shots (prior art). After “n” shots, the region between adjacent mask slits has been completely crystallized by lateral growth. Consequently, ridges form at positions corresponding (approximately) to the centerlines of the adjacent mask features (i.e. slits). Alternately stated, the film region irradiated by a first slit (the first beamlet in the figure) in the laser mask forms a ridge as a result of the final irradiation shot, at the boundary of where an adjacent slit (the second beamlet in the figure) performed its initial irradiation shot. There is a ridge between each area of the film where the different beamlets have performed their final irradiation shot.
It would be beneficial to reduce the size of the ridges formed by the above-mentioned process. It will be even more advantageous to completely eliminate the height variation along the laterally crystallized domain. Such an improvement would relax the positional constraint for the TFT channels formed in an LCD substrate. When roughness (ridges) develops at specific positions 102 (as shown in FIG. 3), the TFT channels need to be arranged to avoid these regions. That is, the TFTs need to be formed in the planar regions between neighboring ridges to avoid performance deterioration. Even more undesirable is the formation of neighboring TFTs with different performance parameters, resulting from the random formation of TFT channels with ridges adjacent TFT channels without ridges. Hence, some sort of alignment is necessary between the crystallized domains and the position of the TFT channels within these domains. This alignment process introduces additional processing steps, hence increases the cost of the process. It would be desirable to eliminate these additional processing steps so that TFT channels can be placed on the processed (laterally crystallized) film without the requirement of calculating ridge alignments.
It will also be desirable if the same process that enables such a surface roughness (ridge) reduction could be used to improve the lateral growth length (LGL) during crystallization. Such improvement would enable an increase of the stepping distance between successive shots. That is, the pitch, or step distance d between shots could be increased (see FIG. 3). The stepping distance of the substrate is a crucial determinant of the process throughput and, hence, in the economics of the LC-ELA process for mass production. The stepping distance depends critically upon the lateral growth length (LGL). LGL is affected by the transient temperature profile of the film, which defines the time possible for the lateral propagation of the two facing crystal fronts, before the remaining molten volume becomes cold enough to trigger copious (explosive) nucleation.